Rename and move gcc4 builds on haiku-file

[pulkomandy]

​http://www.freelists.org/post/haiku-development/Enforcing-gcc2h-was-Alpha-3-better-late-than-never

According to this discussion, there is some confusion because haiku-files doesn't make it clear enough that gcc2hybrid is the only way.

Make the following changes :

• Keep the gcc2hybrid as is on the main page
• Rename the plain gcc2, plain gcc4, and gcc4hybrid images as 'walter' instead of 'haiku'. These are already de-branded inside (splash screen and desktop background).
• Move the gcc4 and gcc4hybrid to a separate page,something like ​http://haiku-files.org/gcc4/anyboot/ (or whatever makes sense on the server)
• Adapt the text on the main page in nightly images paragraph :

 These are regularly built images always using the latest code. While we do try to keep our code in a working state at all times, these are not considered to be stable and may be broken in various ways at any given point in time. They are strictly meant for testing only. Applications made for Haiku should be carefully tested on the latest available stable release.

Official releases leading up to and including R1 Final are distributed only as a GCC 2 Hybrid for the x86 architecture. Other combinations (GCC 2 Only, GCC 4 Only, GCC 4 Hybrid) are available below, for development and testing purposes only. It is recommended to use a GCC 2 Hybrid. 

• Add a section linking to the gcc4/4h/2 stuff below with such wording :

These images are for testing purposes only. They are not compatible with Haiku and shouldn't be used for software development. Unless you are testing for a compiler-dependant bug, please consider downloading a gcc2-hybrid image instead. gcc2-hybrid installations allow to run gcc4-compiled software as well, and they are more tested than these. There is no drawback for using them.

Feel free to improve the texts, but you get the idea.